Hi,



There are several reasons why this can happen.



First of all, if your cat was not tested for Feline Leukemia before the first vaccination was given, he may have been positive all his life. For this reason many vets will not vaccinate a cat against FeLV unless they first test negative for the virus.



Second, a cat can be exposed to the virus, but not test positive unless the virus is in the blood at the time of testing. This can cause a cat to test negative when they are really positive. Occasionally you may see a "weak positive" which means that the cat should be retested in 3 months. It's possible that these cats can actually throw off the virus.



Another thing to consider is the fact that no vaccine is 100% effective. This means that a vaccinated cat if exposed to an infected cat can still get the virus. This would have to be more than a quick, casual, meeting, and is most likely to happen when cats are sharing bowls and litterboxes. A negative cat getting into a fight with a positive cat can also result in the virus being passed on, since it is in the saliva.



Basically, 3 things can happen when a cat is exposed to the Feline Leukemia virus. They may not get sick at all, they can become positive and become ill, or they can be positive and remain healthy. These cats are called healthy carriers and even though they are healthy, they are positive for the virus and can pass it on to other cats.



Occasionally human error can result in a false positive or false negative test result also. This can happen when test kits are not stored properly or there is a mistake made by the person running the test.



I hope all of this information isn't confusing to you. I realize it is a lot, so if you have any other questions, please feel free to ask!



Regards,

PHKitkat

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